This invention relates to satellite communications systems, but more specifically, to a novel method and apparatus for providing on-board control of reconfiguration of interbeam connections in accordance with traffic demands in a multibeam communications satellite system.
In present commercial communications satellite systems operating at 6 and 4 GHz (C-band), 14 and 11 GHz (Ku-band) and 20-30 GHz (Ka Band), on-board connectivity between uplink and downlink beams is carried out on a transponder channel basis by providing "static" switching networks with occasional switch reconfigurations of about fifty to one hundred times per year. Switching schemes employed therein are suitable for carrying Frequency Division Multiple Access (FDMA) continuous traffic. Other satellite systems provide "dynamic" switching with reconfiguration periods of a few milliseconds suitable for carrying Satellite Switched Time Division Multiple Access (SSTDMA) burst traffic. Switching of communications paths between multiple uplink and downlink beams is usually performed by a switch matrix on a transponder channel basis. These on-board switch matrices map input (uplink) traffic into output (downlink) traffic, wherein switch configurations change only the output port connected to a given input port without changing the bandwidths of the respective interconnecting paths. This connectivity will be hereafter referred to as Constant Bandwidth ("CB") connectivity and the associated traffic "CB" traffic.
Present-day CB-FDMA communications systems only employ one-to-one static connectivity networks between uplink and downlink co-frequency transponder channels and utilize mechanical coaxial switches requiring no D.C. power to hold them in position after actuation. A typical switching for this application is a "rearrangeable switch matrix" architecture using "beta" elements as building blocks. On the other hand, present-day CB-SSTDMA (constant bandwidth satellite-switched time-division multiple access) communications satellite systems employ connectivity networks between uplink and downlink channels which utilize coupler cross-bar Microwave Switch Matrices (MSM) of either diodes or field-effect transistors (FETs) having rise-fall times of a few nanoseconds. No on-board satellite-switched TDMA systems having variable bandwidths are presently known to exist.
When on-board interbeam connectivity is represented by a matrix with entries corresponding to the bandwidths of the interbeam connection paths, then a CB connectivity function for one group of co-frequency transponder channels in a satellite system (e.g. 8 beams) at a given time instant is represented by an 8.times.8 matrix having only one non-zero element in each row or column. The following matrix is typical. ##EQU1## where B.sub.T is a transponder bandwidth for the respective channels. In SSTDMA systems, channel configuration is represented by the same matrix [1], but the non-zero matrix elements change their location periodically in time. A complete representation of the entire satellite interbeam connections comprises a number of such switch matrices equal to at least the number of transponder channels.
CB connectivity, in conjunction with the ability to steer spot beams over high traffic sources, so far has proven adequate for heavy route traffic with occasional connectivity changes. Also, for thin-route traffic, CB connectivity may be sufficient. For such traffic, on-board connectivity changes are minimal since carriers are somewhat dispersed in space and time.
Recently, however, in response to traffic demands including a relatively large number of small users, more "intelligent" satellites have been developed which adaptively achieve high satellite design efficiencies via narrow interbeam connectivity paths with reconfigurable bandwidths, e.g., Variable-Bandwidth, Variable Center-Frequency (VBVCF) on-board connectivity. Here, satellite design efficiency is defined as the ratio of saturation capacity to the nominal capacity of the satellite, and provides an indication of how efficiently satellite resources are utilized, such as, how on-board connectivity and antenna coverage match traffic demands.
According to an aspect of the present invention, VBVCF connectivity may be implemented without increasing the number of on-board TWTAs (travelling wave tube amplifiers) by subdividing a transponder bandwidth into a number of narrower channels of varying bandwidth and accommodating within the same transponder different thin route services with different connectivity requirements. Recent on-board TWTA linearization techniques and modulation formats make this design philosophy particularly attractive. As an example, services requiring a continuous band of variable width may be assigned upon demand to a sub-band B.sub.x of transponder bandwidth B.sub.T while the remaining bandwidth B.sub.T -B.sub.x may be channelized into a multiplicity of narrow VBVCF channels suitable for multicarrier traffic with a varying number of different carriers. In its operation, each channel is subsequently routed to a defined downlink beam by a switching network.
A circuit that performs VBVCF demultiplexing and routing functions is called herein an "on-board router". Continuous FDMA traffic with on-board VBVCF connectivity achieved either partially or completely by on-board switching networks is called herein "VBVCF Satellite-Switched FDMA (SSFDMA) traffic". In accordance with a further aspect of this invention, existing CB connectivity is enhanced by providing both CB connectivity and VBVCF connectivity within the same spacecraft wherein the VBVCF connectivity is provided by an on-board router to provide satellite-switch capability.
As discussed herein, prior on-board routers for VBVCF connectivity were proposed as early as 1980, mainly in connection with continuous FDMA traffic. For example, U.S. Pat. No. 4,228,401 entitled "Communication Satellite Transponder Interconnection Utilizing Variable Bandpass Filter" and issued on Oct. 14, 1980 describes a system employing a payload which lacks on-board switching capabilities, but features reconfigurable beam interconnections using VBVCF filters achieved by a serial filter architecture. The VBVCF filter utilized therein performs two successive frequency translations of the signal frequency spectrum with respect to the fixed passbands of two equal filters being serially connected. Unfortunately, this technique, although useful for other applications (see, for example, J. Melngilis and R. C. Williamson, "Filter With Bandwidth Continuously Variable From 5 to 100 MHz", Proc. 1977 Ultrasonics Symp. pp. 965-968), has marginal practical utility for linear phase (constant delay time) communication channels due to the adding up of transmission amplitude and phase ripples in the serially connected passband filters in the vicinity of their upper edge frequency. On the contrary, in the inventive router described herein, the VBVCF demultiplexing function is implemented by a switchable combination of passband filters connected in parallel and does not suffer from the adding up of spectral impurities injected by serial filters.
The on-board FDMA routers proposed in the early eighties mainly relate to 30 and 20 GHz multiple beam satellite systems. At these frequencies, a large frequency spectrum is available for commercial satellite communications (2500 MHz times frequency reuse) and, consequently, prior routers were designed on the basis of broadband channelization schemes which accommodate large numbers of elementary channels which are frequency-multiplexed over large bandwidths. Reconfigurability was achieved by sorting out from a large number of available channels those which closely match user demands. For narrowband reconfigurable connectivity, very large filter banks and switch matrices are present in those routers. Hardware complexity as well as weight and volume render these routers unattractive and caused a shift of interest toward alternate on-board routing solutions such as SSTDMA (Satellite Switched Time Division Multiple Access).
A good summary of on-board SSFDMA router technologies sponsored by NASA is presented in "A comparison of Frequency Domain Multiple Access (FDMA) and Time Domain Multiple Access (TDMA) Approaches to Satellite Service for Low Data Rate Earth Stations", G. Stevens, NASA Tech. Memo. 83430, June 1983. A detailed description of past on-board FDMA router designs is presented in "Non-regenerative Satellite Switched FDMA (SSFDMA) Payload Technologies", P. de Santis, International Journal of Satellite Communications, April-June 1987, Vol. 5, pp. 171-190. Additional information may also be found in J. D. Kiesling, "Study of Advanced Communications Satellite Systems Based on SS-FDMA", G.E. Document No. 80SDS4217 NASA Contract No. NAB-3-21745, May 1980; J. D. Kiesling, "Direct Access Satellite Communications Using SS-FDMA", Proc. AIAA 8th CSSC, Orlando, FL, Apr. 20, 1980, pp. 627-633; and "Customer Premise Service Study for 30/20 GHz Satellite Systems", TRW Space and Technology Group, NASA Contract NAS-3-22889, Final Report, Document No. 038050-011, Apr. 22, 1982.
In view of the present state of the art of on-board technologies and the difficulties experienced in prior satellite routing systems in reconfiguring transponder channels, it is an objective of the present invention to provide a practical routing system for improving the efficiency and flexibility of handling traffic demands.
It is another object of the present invention to utilize advantageously existing on-board CB switching networks to enhance traffic handling capabilities thereof.
It is a more specific object of the present invention to provide a system for reconfigurably dividing a transponder bandwidth into at least two sub-bands of variable width wherein at least one sub-band is further channelized to achieve multiple VBVCF subchannels.
It is a further object of the present invention to provide a system for routing traffic through a demultiplexer with linear phase VBVCF channels which utilizes a "parallel" architecture and is based on a class of multi-level channelization formats preferably implemented by time and frequency multiplexing circuits.
It is yet a further object of the present invention to provide an on-board router with programmable routing of sub-channels to downlink beams wherein each subchannel may accommodate either burst traffic (SSTDMA) or continuous traffic (SSFDMA).